The General Packet Radio Service (GPRS) and Enhanced Data for Global Evolution (EDGE) protocols for the Global System for Mobile Communication (GSM) have introduced a capability of the user data interchange into mobile wireless products. GPRS, and its superset, EDGE, permit efficient use of radio and network resources when data transmission characteristics are packet-based, intermittent and non-periodic, possibly frequent with small transfers of data, for example, less than 500 octets, or possibly infrequent with large transfers of data, for example, more than several hundred kilobytes. User applications that may utilize GPRS/EDGE protocols may include Internet browsers, electronic mail, file transfers, and other applications involving transfers of data.
FIG. 1 is a block diagram of an exemplary GPRS/EDGE communication system 100. Communication system 100 includes multiple Base Station Systems (BSSs) 110, 120 that each provides communication services to a respective coverage area, or cell. Each BSS 110, 120 includes a respective Base Transceiver Station (BTS) 112, 122 that is coupled to a respective Base Station Controller (BSC) 114, 124. Each BSS 110, 120 is coupled to a Serving GPRS Support Node (SGSN) 128, which SGSN is further coupled to a Gateway GPRS Support Node (GGSN) 130 and, via the GGSN, to an external network 132. BSSs 110, 120, SGSNs 116, 126, and GGSN 130 are collectively referred to as a communication system network 140. Communication system 100 further includes an MS 102 that resides in a coverage area, or cell, serviced by a first, source BSS 110 of the multiple BSSs 110, 120. Source BSS 110 provides communication services to MS 102 via an air interface, or wireless communication link, 104 that includes a downlink and an uplink.
As MS 102 moves around in communication system 100, the MS may experience deterioration in radio frequency (RF) signal conditions or congestion conditions with respect to the communication services provided to the MS by source BSS 110. As a result, the MS 102 may decide to perform a cell reselection. In order to facilitate a selection of a new cell, such as a cell associated with BSS 120, MS 102 and network 140 may exchange System Information (SI) messages and measurement information related to the neighboring cells. However, the system provided by the GPRS/EDGE protocols provides only for a discontinuous exchange of such information. That is, such information is exchanged in data packets in GPRS/EDGE systems via a Packet Associated Control Channel (PACCH). The PACCH is a control channel that comprises blocks of four sequential bursts that are stolen from an associated PDTCH. As a result, the PACCH exists only as a portion of a Temporary Block Flow (TBF) comprising data that is being transferred via the PDTCH. When there is no TBF, for example, when a user of MS 102 is receiving rather than sending, the uplink PACCH goes away.
Originally, GPRS/EDGE communication systems were conceived as means of generating additional revenues for system operators based on their excess capacity during non-peak usage period. User applications were originally envisioned to include Internet browsers, electronic mail, file transfers and other applications for which best efforts data transfer are appropriate. At that time, the industry did not anticipate real-time applications, such as streaming and Push-to-Talk (PTT), making use of GPRS/EDGE as an underlying wireless transport vehicle. As a result and in order to maintain system simplicity, no real-time handover procedure is specified for best effort packet data transfers over GPRS/EDGE. Instead, cell reselection is achieved by allowing MS 102 to reselect cells as it does in idle mode, which causes the MS to abort its packet transfer on one cell and completely re-establish the ongoing packet transfer on the new cell. In the meanwhile, until the packet transfer is re-established on the new cell, all data flows are put on hold. As a result, a reselection between cells typically interrupts the flow of data in both directions for 500 milliseconds (ms) to approximately four seconds if the new cell resides in a same Routing Area (RA) as the old cell, that is, if a target BSS is serviced by a same SGSN as a source BSS. If the new cell resides in a different RA, that is, if the target BSS is serviced by a different SGSN that the source BSS, then the impact is typically 8 seconds.
Current metrics show that in an urban area, cell reselection takes place approximately two to four times per minute, even with the MS physically stationary. The lack of a true synchronized handover for a GPRS/EDGE communication system such as system 100 means that the continuity of both uplink and downlink data is compromised each time a new cell is reselected, either autonomously or under the control of network 140. For applications such as PTT, this means that during a one-way conversation, when either a talker or a listener reselects to a new cell, the listener may lose up to eight seconds of voice information and any associated control information conveyed in association with the voice information.
To overcome these problems of data flow disruptions during cell reselection, a true synchronized handover is being considered for the packet domain. However, the introduction of such a packet domain handover is hindered by the fact that there exists no equivalent in GPRS/EDGE communication systems to a Slow Associated Control Channel (SACCH) of circuit-switched communication systems over which handover measurements, typically pilot signal measurements, may flow during, and between, voice data transfers comprising a single application session, such as PTT.
Therefore, a need exists for a method and apparatus that provides for a persistent flow of control data associated with a packet data traffic channel during, and between, packet data transfers comprising a single application session in a GPRS communication system.